In the field of a labelling colors to detect methamphetamine (hereinafter abbreviated as MA), Japanese Unexamined Patent Publication No. HEI3-223673 refers to an MA substituted with a dansyl group. This dansylized MA (hereinafter abbreviated as DNS-MA) emits a 525 nm fluorescence intensity in a 330 nm excited state. DNS-MA bonding to an MA-antibody changes its fluorescence intensity. When DNS-MA encounters a newly added MA molecule, DNS-MA will leave the antibody, reducing the intensity of 525 nm fluorescence. By observing that change, one can carefully detect even a very small amount of MA. However, detection capacity of such a method using DNS-MA is decreased by foreign substances. There are so many naturally-occuring substances having fluorescence around 525 nm that it was difficult to detect MA from such a test sample contaminated with such substances. Thus, a labelling color exhibiting a longer-wavelength fluorescence was needed to better detect MA, even if contaminated.